The present invention relates to a drum base on which a drum of a magnetic tape recorder such as a VTR and a DAT is mounted.
As shown in FIGS. 1 and 2 of the accompanying drawings, a drum 4 of a magnetic tape recorder comprises a lower drum 8 and an upper drum 9. The lower drum 8 is mounted on a drum base 15, which is fixed at a predetermined position of a chassis, not shown. The upper drum 9 is rotatably supported coaxially with the lower drum 8. A magnetic tape 2 is obliquely wound on the peripheries of the drum 4 by tape-guides 10, 11, 12, 13. Specifically, the tape 2 is wound on the lower drum 8 and the upper drum 9. The lower edge of the tape 2 is guided by a step 3 called as a lead formed on the surface of the lower drum 8. For normal recording and reproduction, the tape 2 runs at a predetermined speed in the direction shown by an arrow A. A head 1 is projectingly provided through a gap formed between the lower and upper drums 8, outward from drum surfaces by a predetermined amount. The head 1 rotates coaxially with the drum 9 at high speed in the direction shown by an arrow B. A tip of the head 1 obliquely scans the tape 2 to record signals thereon or reproduce the signals recorded on the tape.
With reference to FIGS. 2-4, the drum base 15 has three annular drum seats 7' formed on its top and three holes 16 formed through it. Each of the seats 7' is coaxial with one of the holes 16. A bottom 6 of the lower drum 8 rests on the seats 7'. The drum 4 is fixed on the drum base 15 by screws 5, which extend through the holes 16 and the seats 7' and are tightened into the lower drum 8 in the direction shown. The seats 7' are positioned on the same plane, and determine the height, inclination and direction of inclination of the drum 4 by coming into contact with the bottom surface 6 of the lower drum 8.
If the working precision of a drum seat 7' is low, as shown in FIG. 4, a projection 17 may be formed. When the drum 4 is mounted and fixed on the drum seat 7' having such projection 17, the bottom surface 6 of the lower drum 8 cannot come into contact with the whole surface of the drum seat 7' and further as a tightening force f of the screw 5 is applied in the direction shown in FIG. 4, a bending moment, which is the product of the tightening force f multiplied by the distance between the projection 17 and the center of the hole 16, is applied. Meanwhile, as magnetic tape recorder drums have been smaller and more lightweight, they have become thinner and less rigid. As a result, the lower drum 8 is distorted locally inward as shown by an arrow C by the bending moment, so that the lead 3 formed on the lower drum 8 is also distorted.
In the example shown in FIG. 4, the projection 17 is located radially outwardly of the drum base 15 with respect to the associated screw 5. If the projection 17 is located radially inwardly of the drum base 15 with respect to the screw 5, the tightening force f of the screw 5 locally distorts the lower drum 8 radially outwardly of the lower drum 8 (reversely to the arrow C), so that the lead 3 is distorted.
If the lead 3 is deformed as shown in FIG. 5A, the tape 2 follows the deformation of the lead 3 as it is guided by the lead 3 and as a result, a recording track 19 formed on the tape 2 becomes wavy and less linear as shown in FIG. 5B.
When a tape on which recording has been made by a recorder with a normal lead is reproduced by a recorder with a deformed lead 3 as shown in FIG. 5A, or when a tape on which recording has been made by a recorder with a deformed lead 3 as shown in FIG. 5A is reproduced by a recorder with a normal lead, the reproduction output of the tracks fluctuates, as shown in FIG. 5C (where the broken lines show normal values). Accordingly, the reproduced wave forms 20 are worse, so that the performance is remarkably low. In addition, under such a condition, part of the signal on an adjacent track may be read, causing cross talk. As a result, the quality of the reproduced signals is remarkably low. Furthermore, the deformation of lead 3 is a factor of unstable tape running or movement, which may prevent signals from being precisely recorded or reproduced.
Japanese Patent Unexamined Publication No. 2-254663 proposes drum base structure for correcting drum deformation. The drum base has seats, and a drum is mounted on the top surfaces of the seats. Each top surface has a linear boundary extending through an axis of an associated hole in the drum base. The extending direction of the boundary is selectively determined so as to correct the drum deformation, when the drum having the drum deformation is mounted on the seats. The publication does not include any disclosure to prevent the drum from being locally deformed when it is fixed to the drum base.